This invention relates to nuclear power plants and in particular to shielding for the nuclear reactor vessel.
In a typical commercial nuclear power plant, a reactor vessel containing the nuclear core is located within a large concrete cavity. Because access for most of the maintenance and refueling operations for the reactor must be from above the vessel, the cavity walls provide shielding only below and laterally of the vessel. Since under all normal operating conditions a substantial amount of water covers the reactor core, additional shielding is not required directly above the core.
There exists, however, a path by which neutrons originating from the reactor core can escape the concrete and water shielding and thereby pose a danger to operators and equipment located above the reactor vessel. This neutron leakage, often called neutron streaming, results from the scattering of neutrons as they emerge from the sides of the reactor core and collide with non-absorbing structures. Such collision paths produce neutrons travelling vertically along the outside of the reactor vessel in the annular space between the vessel and the surrounding cavity wall.
Various techniques have been employed to attenuate and absorb streaming neutrons. For example, one technique seals the annular space between the upper portion of the vessel and the opposite wall with neutron attenuating material cut into large, flat pieces that are fit into the space as pieces would be fit into a jigsaw puzzle. Another type of cavity streaming shield utilizes water bags or water tubs. The water bags have been troublesome because they are subject to irradiation induced deterioration which results in leaking or plugging of their drains. Steel tubs filled with water, like the water bags, require removal and reinstallation during a refueling outage so that the reactor vessel pool seal may be installed temporarily. This critical-path operation not only consumes time, but also increases the radiation exposure of personnel.
Such prior art techniques also have deficiencies associated with the consequences of an hypothesized loss of coolant accident, whereby a primary nozzle or pipe break injects high temperature, high pressure steam into the space between the vessel and the cavity wall. The techniques which effectively seal the space above the nozzles with the shielding material render the annular space a sealed chamber in which extremely high pressures can build up and produce imbalanced loads on the reactor vessel. Also, as the pressure builds up to a threshold value, the shield will finally blow off and send shield fragments rocketing into the containment building where additional damage may be done.
Another major disadvantage is that the prior art shields, by sealing off the annulus, prevent satisfactory heat dissipation from the external surfaces of the reactor vessel. If the prior art techniques were to leave spaces or openings to allow the free flow of heated air up the annulus (or to minimize pressure peaks in the event of an accident), they would also allow some unwanted streaming of neutrons through the shield.